Hi, I've been with DC in the UK for a couple of years.
Don't expect any maths in either of the rounds - maybe just some percentages, or being able to spot trends/anomolies from a table of numbers
DC (soon to become Braxton - make sure you know all the recent history) is very personality focused, and prides itself on its 'collegiality' i.e. working with a client, rather than doing it to a client. Therefore, just be yourself: show you understand consulting and sell yourself on your varied and interesting experiences.
When you get to the second round, you'll need to show you have confidence and the skills to recognise and handle group dynamics (but not necessarily assume leadership).
Best case study web-page I ever found was from Boston Consulting Group - but American based: